1. Field of Invention
The present invention relates to a drive system for vehicles with at least two drivable vehicle axles, wherein the drive system has an infinitely variable transmission with no intermediate axle differential comprising at least a first and a second motor, wherein the first motor is propulsively connected to a first vehicle axle and the second motor is propulsively connected to a second vehicle axle, and a clutch by which a propulsive connection can be made between at least the first and the second vehicle axle. Furthermore the present invention relates to a method for operating such a drive system.
2. Description of Related Art
Utility vehicles, particularly agricultural utility vehicles, such as for example a farm tractor, frequently have at least two drivable vehicle axles. Also passenger cars, which are designed to travel over wet or smooth roadways and off-road, frequently have two drivable vehicle axles. In the case of such vehicles and particularly over rough terrain, all vehicle axles and thus all wheels can be driven, so that—as is generally known for all-wheel drives—traction is improved as a result.
The German Patent DE 42 09 950 C2 describes a drive system for two-axled vehicles used in agriculture and civil engineering. With this drive system each vehicle axle is assigned at least one hydraulic motor. The two hydraulic motors are driven by a hydraulic pump. The hydraulic pump is driven in turn via a hydrostatic-mechanical power branched transmission by a combustion engine. The hydraulic motors are adjustable independently from one another and thus allow variable rotational speed and torque distribution between front axle and rear axle of the vehicle. Furthermore a switchable clutch enables the front axle and the rear axle of the vehicle to be propulsively connected, so that the two vehicle axles can be driven at a constant rotational speed ratio with the common torque of both hydraulic motors. The switchable clutch is engaged particularly with extremely unequal axle load distribution, so that up to 100% of the possible transmission output torque can be transmitted via a vehicle axle. Thus higher torque than can be made available by the hydraulic motor alone which is propulsively connected to this vehicle axle, is transmitted via a vehicle axle with engaged clutch.
The European Patent EP 1 364 824 A2 describes a control unit for an intermediate axle differential transmission of a vehicle. The intermediate axle differential transmission, which serves to transmit torque to the front wheels and to the rear wheels, comprises an actuator, in particular a tuneable modulating clutch, which adjusts a ratio between the front wheel speed and the rear wheel speed. The actuator in this case is controlled as a function of a steering angle of the front wheels, which is detected by a steering angle sensor.
The intermediate axle differentials, used in road vehicles to obtain a variable translation between the vehicle axles, are usually formed in such a manner that the torque distribution between the vehicle axles is permanently pre-set for all driving speeds and operating conditions. In the case of utility vehicles and farm tractors when traveling over rough terrain extremely unequal axle load distributions develop in some instances. Due to the permanently pre-set torque distribution of these intermediate axle differentials, sufficiently high torque in many cases cannot be supplied to the respective vehicle axle, at which increased traction power demand exists. Accordingly the intermediate axle differentials, often provided in road vehicles, are not suitable for use in vehicles with frequently widely varying axle load distribution, such as for example traction engines and farm tractors.
A drive system, which comprises several hydraulic or electric motors, wherein a motor is propulsively connected to an associated vehicle axle in each case, equally allows variable translation between the vehicle axles. As described in the German Patent DE 42 09 950 C2 for example, the respective motors may be adjustable, so that the torque output of the individual motors can be varied as a function of the traction power demand. The traction power on a highly loaded vehicle axle can be increased by such splitting of the torque output of the individual motors but only up to the maximum torque of the motor, which is propulsively connected to this vehicle axle. Corresponding dimensioning of a step-less drive for each vehicle axle in such a way that traction power can be supplied to this sufficient for utility and agricultural vehicles is not generally possible on the grounds of construction space, efficiency and cost. Therefore utility vehicles, as described in DE 42 09 950 C2 for example, are frequently equipped with switchable clutches, which selectively allow a rigid propulsive connection between the two vehicle axles of the vehicle by engagement of the clutch.
By engaging the switchable clutch and by making the rigid propulsive connection between the vehicle axles however the advantages of variable rotational speed and torque distribution between the vehicle axles, as is possible with disengaged clutch, must be forfeited. Particularly when rounding tight bends the front wheels of the vehicle cover a greater distance than the rear wheels, so that with engaged clutch and thus when there is a rigid propulsive connection between the vehicle axles, tensions in the power train arise.
Accordingly the present invention is based on the object of producing a drive system, of the type indicated above, for vehicles with at least two drivable vehicle axles, by means of which high traction power can be supplied to individual vehicle axles when required and wherein generally the advantages of variable rotational speed and torque distribution between the vehicle axles remain in place.
The object is solved by a drive system according to claim 1 as well as by a method for operating a drive system according to claim 15. Further embodiments of the invention are indicated in the sub-claims.